std::future<T>::get

Since C++11

The get member function waits (by calling wait()) until the shared state is ready, then retrieves the value stored in the shared state (if any). Right after calling this function, valid() is false.

# Declarations

Main template

T get();

(since C++11)

std::future<T&> specializations

T& get();

(since C++11)

std::future<void> specialization

void get();

(since C++11)

# Notes

The C++ standard recommends the implementations to detect the case when valid() is false before the call and throw a std::future_error with an error condition of std::future_errc::no_state.

# Example

#include <chrono>
#include <future>
#include <iostream>
#include <string>
#include <thread>
 
std::string time()
{
    static auto start = std::chrono::steady_clock::now();
    std::chrono::duration<double> d = std::chrono::steady_clock::now() - start;
    return "[" + std::to_string(d.count()) + "s]";
}
 
int main()
{
    using namespace std::chrono_literals;
 
    {
        std::cout << time() << " launching thread\n";
        std::future<int> f = std::async(std::launch::async, []
        {
            std::this_thread::sleep_for(1s);
            return 7;
        });
        std::cout << time() << " waiting for the future, f.valid() = "
                  << f.valid() << '\n';
        int n = f.get();
        std::cout << time() << " f.get() returned " << n << ", f.valid() = "
                  << f.valid() << '\n';
    }
 
    {
        std::cout << time() << " launching thread\n";
        std::future<int> f = std::async(std::launch::async, []
        {
            std::this_thread::sleep_for(1s);
            return true ? throw std::runtime_error("7") : 7;
        });
        std::cout << time() << " waiting for the future, f.valid() = "
                  << f.valid() << '\n';
 
        try
        {
            int n = f.get();
            std::cout << time() << " f.get() returned " << n
                      << ", f.valid() = " << f.valid() << '\n';
        }
        catch (const std::exception& e)
        {
            std::cout << time() << " caught exception " << e.what()
                      << ", f.valid() = " << f.valid() << '\n';
        }
    }
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 2096	C++11	overload (1) needed to check whether T is MoveAssignable	not required

# See also

valid